The hand is one of the most complex, versatile, and critical human sensory organs. Sensory signals from the hand play a key role in guiding the dexterous manipulation of objects. Without these signals, we would need to constantly watch our hand to ensure that an object was properly grasped, to monitor our use of a tool, and to maintain its proper shape. Furthermore, simple activities of daily living, such as tying one's shoe, or turning a door knob, would be slow and clumsy, and require great concentration. In contrast to the sense of touch, which has received a lot of experimental attention, the representation of hand position and hand movements in the brain is poorly understood. Indeed, unlike vision and audition, for which monitors and speakers are commercially available, proprioception research presents unique technological challenges in measuring and manipulating the sensory input. As increasingly sophisticated technologies become available, however, we are able to more fruitfully investigate this important sensory modality. Our objective is to apply rigorous and quantitative methods to investigate the cortical basis of proprioception in primates, with an emphasis on hand kinematics and posture. Furthermore, we seek to understand how proprioceptive representations interact with motor representations in the cortex to guide behavior. To these ends, monkeys will perform two tasks: a grasping task, with objects different widely in shape and size, and a finger-press task, in which the animal is instructed to make individuated finger movements. While the monkeys perform these tasks, we will measure their hand movements using a camera- based motion tracking system, and record, using chronically implanted high-density multi-electrode arrays, the neuronal activity evoked in proprioceptive and motor areas of the cortex. We will then use a variety of mathematical techniques to (1) understand how hand movements and position are represented in the activity of individual neurons and of neuronal populations and (2) characterize the interactions between proprioceptive and motor representations to understand the role of this sensorimotor interplay in motor control. Our labs are currently developing approaches to convey sensory feedback through electrical stimulation of cortex for use in upper-limb neuroprostheses. Proprioceptive information about the position and movements of the prosthetic limb is essential for a complete prosthetic limb for the same reasons it is important in the native limb. Results from the proposed project will thus inform how to incorporate proprioceptive feedback in sensorized neuroprostheses.